This invention relates to apparatuses for controlling pests such as insects and rodents and, more particularly, to such apparatuses which operate to repel pests by generating ultra-sonic sound at frequency levels which cannot be tolerated by the pests.
It is known that pests can be controlled by using ultra-sonic means for producing sound waves at levels which cannot be heard by humans, but which is repugnant to the sound detection systems of insects and rodents. One early example of such a device is shown in U.S. Pat. No. 3,138,138.
Many different devices have been developed which use different types of ultra-sonic wave forms and equipment for generating the sound. For example, randomly varying signals are generated in devices taught by U.S. Pat. Nos. 4,186,387, 3,872,472 and 3,636,559 and an alternating signal in U.S. Pat. No. 3,058,103. Unijunction transistors are used in U.S. Pat. Nos. 3,891,962 and 3,886,492 for continuously varying frequencies produced from an output signal with a sawtooth wave form. Multiple frequencies are used simultaneously in U.S. Pat. Nos. 4,178,578 and 3,503,039. A plurality of speakers are used in U.S. Pat. No. 3,873,960 and a specific disk reflector is taught in U.S. Pat. No. 3,879,702.
An improvement over the above-mentioned U.S. Pat. No. 3,636,559 is taught in U.S. Pat. No. 3,838,418 where instead of randomly modulating the swept signal taught in that former patent, the signal is broken into periodically repeating sawtooth ramps with sloping amplitudes and contiguous alternate segments with constant amplitudes so that successive tone generator outputs are broken by a silent period and have continuously changing frequencies.